This invention relates generally to test instrumentation, and in particular to techniques for detecting bridge tap and other types of fault in a transmission line using frequency domain analysis.
Transmission lines are ubiquitous, and are use for transmission of communications and other electromagnetic signals. Examples of transmission lines include telephone wires, power lines, coaxial cables, twisted wire pair, and others. Typically, before a transmission line is activated for use, testing is performed to qualify the line. Such testing often includes the detection of faults (or events) corresponding to discontinuities in the impedance of the transmission line.
A common type of fault in a transmission line is a bridge tap. A bridge tap is a popular mechanism for attaching additional circuits to a transmission line, and comprises an additional line (also referred to as a lateral) coupled or spliced to the main line. The presence of the bridge tap affects the characteristic impedance of the transmission line to which it couples. The change in the line impedance due to the bridge tap is dependent on many factors, such as the length of the bridge tap, the circuitry coupled to the bridge tap (i.e., the loading), the characteristic impedance of the bridge tap, and other factors.
A transmission test set can be used to detect faults in a transmission line. More specifically, a time domain reflectometer (TDR) is conventionally used to detect discontinuities in the line impedance by transmitting pulses of energy, measuring the reflected pulses (if any), and determining the type and location of the fault(s) by analyzing the time-domain xe2x80x9csignaturexe2x80x9d of the reflected pulses. The reflections are caused by both expected faults (e.g., gauge changes, splices) and unexpected faults (e.g., shorts, opens, water) in the transmission line.
The magnitude and phase of the reflected pulses are determined by the characteristics of the particular faults along the transmission line. For example, if the fault is an open circuit, the reflected pulse is in-phase with the transmitted pulse (i.e., both pulses have the same polarity). Alternatively, if the fault is a short circuit, the reflected pulse is out-of-phase with the transmitted pulse (i.e., the pulses have opposite polarities). Thus, by analyzing the magnitude and phase of the reflected pulses, an estimate can be made as to the identity and location of the faults in the transmission line.
The testing of a transmission line for faults is made challenging by a number of additional factors. For example, a transmitted signal in a transmission line naturally exhibits attenuation (or loss) due to the resistive, inductive, and capacitive characteristics of the transmission line. This natural attenuation degrades and distorts the transmitted and reflected signals, thus making it more difficult to accurately identify faults. The transmission line loss, particularly for a twisted wire pair, is also worse at higher frequencies, which tends to mask details in the reflected pulses.
From the above, techniques that can accurately detect the presence of faults, such as a bridge tap, in a transmission line is needed in the art.
The present invention provides techniques for detecting the presence of faults, including bridge tap, in a transmission line using frequency domain analysis. Initially, a frequency response is obtained for the transmission line under test. The frequency response is then analyzed for the presence of frequency-domain xe2x80x9csignaturesxe2x80x9d that are indicative of specific types of fault. For example, a frequency-domain signature comprising a set of harmonically related attenuation dips is indicative of the presence of a bridge tap in a transmission line. Each attenuation dip corresponds to a local minima in the frequency response. Other frequency-domain signatures can be associated with other types of fault.
An embodiment of the invention provides a method for detecting the presence of bridge tap and other types of fault in a transmission line. In accordance with the method, test signals of predetermined frequencies are transmitted into a transmitting end of the transmission line. The test signals are received at a receiving end of the transmission line and the amplitudes of the received signals are measured. In an embodiment, the transmitting and receiving ends are opposite ends of the transmission line. A frequency response of the transmission line is computed based on the measured amplitudes. The frequency response is then analyzed for the presence of a frequency-domain signature that corresponds to one of the detectable types of fault. The presence of a bridge tap or another type of fault is identified based on the identified frequency-domain signature. The frequency-domain signature associated with a bridge tap can comprise a set of one or more (i.e., harmonically related) attenuation dips in the frequency response, with each attenuation dip corresponding to a local minima in the frequency response.
The length of the bridge tap can be estimated based on the frequency of the first attenuation dip in the set of attenuation dips. Also, the location of the bridge tap in the transmission line can be estimated by performing a time domain reflectometer (TDR) test.
The invention also provides a computer program product that implements the method described above.
Another embodiment of the invention provides a test set for detecting the presence of bridge tap and other types of fault in a transmission line. The test set includes at least one signal input port, test circuitry, a processor, and a display. The test circuitry couples to the at least one signal input port and is configured to receive signals from the signal input port and generate data indicative of the amplitudes of the received signals. The processor couples to the test circuitry and is configured to receive the data indicative of the amplitudes of the received signals and generate a frequency response. The display operatively couples to the processor and is configured to receive and display the frequency response (i.e., in tabular or graphical form). The frequency response is analyzed for the presence of a frequency-domain signature that identifies the presence of a bridge tap or another type of fault in the transmission line. Again, the frequency-domain signature associated with a bridge tap can comprise a set of one or more (i.e., harmonically related) attenuation dips in the frequency response. The test set can further include a user-input device that couples to the processor and is configured to receive a signal indicating the frequency of an attenuation dip in the frequency response. The processor is then further configured to compute the length of the bridge tap based on the indicated frequency of the attenuation dip.
The foregoing, together with other aspects of this invention, will become more apparent when referring to the following specification, claims, and accompanying drawings.